For millennia, gold has captivated humanity with its allure and value. But beyond its beauty and economic significance, a fundamental question persists: where did gold, and other precious metals, actually come from? New research is shedding light on this very question, offering a compelling theory about the cosmic origins of gold found in the Earth’s mantle.
The prevailing scientific understanding is that Earth’s story is intertwined with dramatic cosmic collisions. Early in our planet’s history, it endured bombardment from massive, moon-sized objects in space. These impacts, while cataclysmic, are now believed to be the delivery mechanism for many elements, including the precious metals we treasure today. However, the specifics of how these metals migrated to accessible parts of the Earth’s mantle have remained an enigma, until now.
The Puzzle of Precious Metals on Earth’s Surface
Gold, platinum, and similar elements are classified as highly “siderophile.” This scientific term means they have a strong affinity for iron. Given Earth’s composition, with a dense iron core, conventional wisdom suggested that these metals, upon arrival via asteroid impacts, should have sunk directly into the Earth’s core. Either they would have merged with the core during the initial impact or gradually descended from the mantle into the core due to their density and iron-loving nature.
This expectation created a significant conundrum: if precious metals are so drawn to iron and should be deep within the Earth’s core, why are they found in relatively shallow pockets within the mantle, accessible enough for mining and discovery? This is the puzzle that researchers from Yale University and the Southwest Research Institute (SRI) have tackled.
Space Collisions and Earth’s Gold
Scientists have long established the link between space collisions and the arrival of precious metals on Earth. Billions of years ago, in the chaotic early solar system, proto-Earth was subjected to intense bombardment. These impacts delivered a variety of materials, including siderophile elements. The challenge was to explain how these elements defied their expected behavior and remained in the mantle instead of disappearing into the core.
The Transient Mantle Region: A New Theory
Jun Korenaga, a professor at Yale, and Simone Marchi, a researcher at SRI, have proposed a novel solution centered on a “transient” region within the Earth’s mantle. Their theory, published in the Proceedings of the National Academy of Sciences, highlights a critical, short-lived zone that forms in the mantle after a major impact. This region is characterized by a partially melted upper layer and a solid lower layer.
How the Transient Region Traps and Delivers Gold
Korenaga and Marchi’s research suggests that this transient region possesses unique dynamic properties. It acts as a trap for the metallic components delivered by impacts. Instead of immediately sinking to the core, these metals become caught within this zone. Over time, this transient region slowly mixes with the rest of the mantle, effectively distributing the precious metals throughout it.
Intriguingly, the researchers suggest that remnants of these ancient transient regions may still exist deep within the Earth, manifesting as “large low-shear-velocity provinces” – geophysical anomalies detected in the deep mantle. This implies that the process of precious metal delivery and distribution is not just a historical event but could still be subtly ongoing.
Implications and Broader Context
This new theory has significant implications for our understanding of Earth’s geochemical and geophysical evolution. It offers a compelling explanation for the seemingly paradoxical presence of precious metals in the mantle. Furthermore, it underscores the vast timescales involved in planetary formation. The transient mantle region, while existing for a geologically brief period of about a day after an impact, has had a lasting influence on Earth’s composition over billions of years.
The research not only refines our understanding of where gold comes from on Earth but also provides broader insights into planet formation across the universe. By re-examining established ideas, scientists continue to uncover unexpected discoveries, deepening our knowledge of the cosmos and our place within it.
